1. Field of the Invention
The present invention relates to motion predicted image signal compression.
2. Description of the Prior Art
A motion predicted, inter frame, image signal compression system is known. An example of such a system is MPEG-2 as set out in the Motion Pictures Expert Group II standard ISO/IEC Publication DIS 13818/2 “Information Technology—generic coding of motion pictures and associated audio information, March 1995. Such a system uses Discrete Cosine Transformation (DCT) of image data as one of several compression techniques.
To additionally compress the image data, motion prediction is used, in which there is calculated in the space domain the position in a reference frame of an image block, which image block (called the search block) occurs in a succeeding frame. That is done by comparing the search block with similar size blocks in the reference frame until a match (if any) is found. In place of the image information of the search block, only the position of the matching block in the reference frame is used. The image information of the search block is then derived from the matching block in the reference frame. The matching block in the reference frame is used as a prediction of the search block. The position information so produced is termed a “motion vector”.
It has been proposed to implement a motion-compensated inter-frame compression system using another known transform such a Wavelet Transform or a Sub-Band Transform in place of DCT.
When a Wavelet Transform or a Sub-Band Transform is used, an input image is transformed into two dimensional spatial frequency bands, each of which is a differently sub-sampled version of the input image.
It has been proposed that frequency domain motion prediction is carried out on the frequency transformed data. To do that a search block is defined for each frequency band. For each frequency band the search block is compared to correspondingly sized blocks in a reference frame. Thus for each frequency transformed image frame as many motion predictions are needed as there are frequency bands.